The present invention relates to force sensors. More specifically, the invention relates to multi-range force sensors manufactured with shape memory alloys to improve force measurement sensitivity.
Robotic and industrial applications generate contact forces between themselves and the objects of their manipulation during the performance of their activities. These contact forces, typically include forces generated by the payloads and those generated during the alignment and attachment of parts during an assembly. A system of control is necessary to monitor these contact forces so as to allow adjustments to the robotic or industrial tool to quality performance or successful job completion. The control system used typically involves repositioning of the end effector during the application in response to force and torque measurements derived from a force sensor, or load cell. For example, in an assembly operation where a robotic manipulator is used to align or affix a part, a force sensor on the manipulator will measure the contact forces generated between the part and the assembled product. The manipulator will then use the force measurement to automatically align the part or release the part once it is in its desired position.
The most prevalent type of force sensor is the strain gauge load cell. The strain gauge load cell is a force sensor which utilizes strain gauges to measure the induced strain (flexure) placed upon a sensing element (load cell) by an applied force. The load cell serves as the reaction center for the applied force and functions by focusing the force into isolated and uniform strain fields where strain gauges are attached. The strain field, in turn, causes a change in the resistance across a strain gauge which is measured according to the modulation of an output signal across an electrical wheatstone bridge circuit, of which the strain gauge is a part. The output signal is then conditioned by dedicated electronic filters to eliminate any electronic noise, and gains are used in both computer hardware and software to obtain a measurable signal to determine the force, or load, applied during the application.
The ability of a strain gauge load cell to provide an accurate force measurement depends upon the sensitivity of the load cell to the applied force and the ability of the load cell to react to this force in a linear-elastic manner. The sensitivity of the load cell, in turn, depends upon the load cell""s underlying elasticity which dictates the extent to which the load cell will react to the contact force and produce a measurable strain. Accordingly, a load cell designed to measure small contact forces is highly elastic and will allow flexure under minimal forces. A load cell designed to measure large contact forces, on the other hand, will allow flexure only under heavy forces and will have minimal strain under small forces. However, the low strain for small forces also limits the ability of the force sensor to measure small contact forces. In this case, the flexure induced by the small contact force is minimal and generates a change in resistance across the strain gauge which is too small for accurate measurement.
Robotic manipulators used in assembly operations, however, are generally required to carry varying payloads or carry out variable tasks which result in the generation of a wide range of contact forces. For example, the initial contact force applied to a robotic manipulator while grasping a part is generally much higher than the force required to insert and position the part during the assembly process. These contact forces need to be measured to allow precision robotic manipulations. Under the current technology, the measurement of these forces will require the use of many sensors having various capacities or measuring ranges.
One possible solution is to use a heavy force sensor and make adjustments in either the computer""s software or hardware gains to obtain a full-scale reading of the electronic signal generated by the small contact force. Unfortunately, as the magnitude of the signal is improved, its accompanying noise is equally magnified, thus impairing an accurate reading. As such, increasing the gain will not improve the sensitivity of the heavy force sensor to measure small contact forces.
Other commercial sensors have attempted to overcome the problems associated with measuring forces over wide ranges by developing forces sensors which allow the interchanging of load cells within the sensor according to the applied load. For example, U.S. Pat. No. 5,471,885 describes a force sensor which utilizes freely interchangeable pre-calibrated load cells having various capacities. The pre-calibrated load cells are housed in a load cell module which is readily attached by screws to a display module which provides the measurement of the force applied to the load cell. The use of this technology requires an individual to physically detach the load cell from the display module and then attache a new load cell module having the desired capacity. In robotic and industrial applications, this is not practical.
The present invention uses shape memory alloys-to overcome the problems associated with measuring contact forces over extended ranges. Shape memory alloys are materials that exhibit a phase transformation and an accompanying mechanical property transformation dependant upon the temperature of the material. When the alloy""s temperature is lowered below the alloy""s martensite finish temperature, TMf, its particular crystal structure is changed, placing the alloy in a low strength state such that the alloy is more pliable. At this low temperature, low strength state the alloy is said to be in its martensite phase. Upon heating the alloy to its higher austenite finish temperature TAf, the alloy reverses its crystal structure and manifests a memory effect by resuming its original shape imposed upon the alloy during a training phase. At this high temperature, high strength state the alloy is considered in its austenite phase. The transition of the alloy in moving from its martensite phase to its austenite phase is considered phase transformation. The shape that the alloy achieves during its austenite phase derives from the firing of the alloy in the shape at a temperature range well in excess of the TAf The temperature for this training phase will depend upon the particular properties of the material.
Shape memory alloys are known to have commercial use in force actuators, free-recovery mechanisms and proportional controllers. Actuators function by taking advantage of the shape change of the shape memory alloy during phase transformation. Actuators generally operate by deforming a shape-memory alloy while it is in its martensite phase and then heating the alloy to its austenite phase to recover all or part of the deformation, and in the process creating a motion of one or more mechanical elements. For example, U.S. Pat. No. 5,061,914 discloses the use of shape memory alloys in micro-actuators to open and close valves, move switches and providing motion for micro-mechanical devices. Likewise, U.S. Pat. No. 4,899,543 discloses the use of a shape memory alloy actuator as part of a clamping device to apply a compressive force on a work object while in an intermediate position under normal temperatures and then heating the actuator to extend the clamping device and release the compressive force.
Another device has taken note of the fact that the force generated by the actuator varies during its transformation phase and has attempted to measure that force according to the actuator""s temperature and displacement. U.S. Pat. No. 4,579,006 discloses a force sensing means for an actuator which is made of a shape memory alloy. In this system, the actuator is coupled to a driver which is capable of heating the shape memory alloy of the actuator through its transformation phase so as to drive a load coupled to the actuator through a force transmission unit. A force sensing means is included which determines tensile force information according to a formula which considers the temperature or resistance of the actuator during its transformation phase and its displacement along the transmission unit.
Unfortunately, this system is not applicable to modem day robotic and industrial applications. For example, the disclosed sensing means is limited to only measuring tensile loads and is unable to measure compressive or shear forces. Moreover, this system is not adaptable to measure contact forces over a wide range of force levels with the requisite accuracy. The determination of the applied force is also dependent upon the analysis of the shape memory alloy while it is in the transition temperature range between the martensite and austenite phases such that the hysteresis experienced during phase transformation is likely to effect the reading of any force sensor output. Finally, the use of external contact and non-contact displacement sensors as suggested will require the use of bulky equipment which may be inappropriate for today""s force sensors.
It is an object of the present invention to provide a force sensor which is capable of measuring a wide range of contact forces without experiencing a reduction in sensitivity to low level contact forces.
It is another object of the present invention to provide force sensors which are small in size and light weight such that they are capable of use in several different applications, including modern day robotic and industrial applications.
It is yet another object of the present invention to provide force sensors capable of enduring shock loads or overloading.
These and other objectives are accomplished by the present invention. The present invention is summarized in that it provides a multi-range force sensor comprising a load cell made of a shape memory alloy, a strain sensing system, a temperature modulating system, and a temperature monitoring system. The ability of the force sensor to measure contact forces in multiple ranges is effected by the change in temperature of the shape memory alloy. The temperature modulating system functions to place the shape memory alloy of the load cell in either a low temperature, low strength martensite phase for measuring small contact forces, or a high temperature, high strength austenite phase for measuring large contact forces. Once the load cell is in the desired phase, a strain sensing system is utilized to obtain the applied contact force. The temperature monitoring system is utilized to ensure that the shape memory alloy is in one phase or the other.
In one embodiment of the present invention, the force sensor comprises a load cell made of a shape memory alloy, one or more thermoelectric devices for adjusting the temperature of the load cell, one or more thermocouples for monitoring the temperature of the load cell, and one or more strain gauges for measuring the applied contact force.
One advantage of the present invention is that it provides a force sensor which is capable of measuring a wide range of tensile forces, compressive forces and shear forces.
Another advantage is that the present invention provides a force sensor which is sensitive and highly accurate to low level contact forces while capable of providing precision measurements over a wide range of contact forces.
Yet another advantage of the present invention is that the force sensors maintain an inherent form of shock prevention in that the shape memory alloys utilized in the load cells have a unique propensity to return to their original state after experiencing deformation caused by shock loads or overloading. The recovery from the deformation arises by heating the alloy to its austenite phase and then cooling the alloy back to its martensite state.